There is known from Patent Document 1 below a belt type continuously variable transmission in which an endless belt is wound around a drive pulley provided on an input shaft and a driven pulley provided on an output shaft, and gear shifting is carried out by changing the groove width of the two pulleys by gear shift hydraulic pressure, wherein power transmission efficiency is improved by controlling pulley lateral pressure applied to the pulleys based on parameters such as a slip identifier IDslip and a phase lag Δφ, which are introduced while noting the occurrence of a difference in amplitude or phase between a variable component of the input shaft and a variable component of the output shaft due to slip of the endless belt.